Many applications require small, light weight, efficient conformal antennas. Traditionally, microstrip patch antennas have been preferred where only a narrow frequency band is used, since microstrip patch antennas typically are efficient only in a narrow frequency band. Advantages of these antennas include their capability of being mounted in a small space, of having high gain, and of being constructed in a rugged form. Such advantages have made them the antennas of choice in many applications.
In contrast to the narrowband performance of conventional microstrip patch antennas, satellite communication (Satcom) systems and other similar communications systems, require antennas that are functional across a relatively broad band of frequencies. Typical military broadband applications include long range communication links for smart weapon targeting and real time mission planning and reporting. A variety of antenna designs, such as crossed slots, spirals, cavity-backed turnstiles, and dipole/monopole hybrids have been used for similar applications over at least the last 15 years. However, most of these broadband antennas require large installation footprints. Particularly, a typical UHF antenna requires a square which is two to three feet on a side. When used on aircraft, these antennas intrude into the aircraft by as much as 12" and can protrude into the airstream as much as 14". For airborne Satcom applications, antennas of this size are unacceptably large, especially on smaller aircraft, and difficult to hide on larger aircraft, where it is undesirable to advertise the presence of a UHF Satcom capability. Therefore, there has been a need for highly efficient broadband antennas having the size, weight, and durability advantages provided by narrowband microstrip patch antennas.
Of further concern, in Demand Assigned Multiple Access (DAMA) operations, for example, UHF Satcom antenna systems require switching times between frequencies of as fast as 875 microseconds. Accordingly, an antenna system for use in such operations, as well as in TDMA and other frequency hopping applications, must be compatible with such requirements and must include control circuitry that can configure the broadband antenna with minimal delay.
Moreover, various operating conditions can alter the performance characteristics of a microstrip antenna. For example, temperature on a microstrip patch substrate can change the resonant frequency of the patch, causing the antenna to be improperly tuned. Accordingly, an antenna system should include control circuitry that can monitor such operating conditions and configure the antenna to account for them.